Large-Scale Multifunctional Electrochromic-Energy Storage Device Based on Tungsten Trioxide Monohydrate Nanosheets and Prussian White

Bridging to Commercialization: Record-Breaking of Ultra-Large and Superior Cyclic Stability Tungsten Oxide Electrochromic Smart Window

Electrochromic smart windows (ESWs) can significantly reduce energy consumption in buildings, but their cost-effective, large-scale production remains a challenge. In this study, the instability of black phosphorus is leveraged to induce the growth of the tungsten oxide film through its decomposition process, inspired by the 2D material-assisted in situ growth (TAIG) method. This approach results in the preparation of large-scale, high-performance WO3-x·nH2O (n < 2) films. Characterization techniques and DFT calculations confirm efficient regulation of structural water and oxygen vacancies during TAIG preparation. The WO3-x·nH2O films exhibit excellent electrochromic (EC) properties, including high transmittance modulation (74.2%@1100 nm), fast switching time (tc = 5.5 s, tb = 3.8 s), high coloration efficiency (124.7 cm2 C-1), and superior cyclic stability (transmittance modulation retained 94.7% after 20 000 cycles). Ultra-large WO3-x·nH2O film are prepared via a simple immersion process, and fabricated into a large-area ESW under facile laboratory conditions, demonstrating the economic and practical feasibility of this approach in industrial-scale production. Operated by the intelligent control circuit,  the ESW exhibits remarkable EC properties and cyclic stability This research represents a milestone in improving the performance and industrial-scale production of ESWs, bridging the gap to the commercialization of EC technology.

An ITO free All Tungsten-Based Electrochromic Energy Storage Device as Smart Window

Excessive electricity usage in buildings, notably for heating and cooling, accounts for over 30% of energy consumption, creating a pressing need for energy-saving solutions. Electrochromic Smart Windows (ECSW) aims to reduce energy use while maintaining comfort but faces high costs due to materials like tin-doped indium oxide (ITO) and thick electrochromic films. Moreover, achieving full opacity in the colored state of ECSW is a bottleneck for the industry to overcome privacy concerns. Herein, efforts are directed toward finding cost-efficient alternatives, with all-tungsten-based mesh networks showing promise due to enhanced stability. This newly developed ITO-free, all-tungsten ECSW displays minimal transmittance (≈3%) in the colored state using only 260 nm thick sub-stoichiometric tungsten oxide (WO3-x) film within a lithium-ion-based electrolyte. The ECSW device of size (25 cm2) also demonstrates areal capacitance of ≈13 mF cm-2 to power a liquid crystal display (LCD) for ≈25 min, showcasing its energy storage capabilities. Additionally, to confirm scalability and cost-effectiveness, a larger 15 × 15 cm2 ECSW utilized a single hybrid electrode, highlighting the potential for reducing costs when scaling up production processes. This advancement represents a significant stride toward accessible and energy-efficient smart window technology, offering broader applicability within modern architectural practices.

Utilizing Natural Materials in Electronic Devices: Inching Toward "Herbal Electronics"

Sustainable development is the primary key to address global energy challenges. Though the scientific community is engaged in developing efficient ways to not only maximize energy production from natural resources like sun, wind, water, etc. but also to make all the electronic gadgets power efficient, despite all this, the materials used in most of the electronic devices are largely produced using various materials processing techniques and semiconductors, polymers, dielectrics, etc. which again increases the burden on energy and in turn affects the environment. While addressing these challenges, it is very important to explore the possibility to directly, or with minimum processing, utilize the potential of natural resources in the development of electronic devices. Recent articles are focused on the development of herbal electronic devices that essentially implement natural resources, like plants, leaves, etc., either in their raw or extracted form in the device assembly. This review encompasses the recent research developments around herbal electronic devices. Furthermore, herbal electronics has been discussed for several functional applications including electrochromism, energy storage, memresistor, LED, solar cell, water purification, pressure sensor, etc. Moreover, advantages, disadvantages, and challenges encountered in the realization of "herbal electronics" have been discussed at length.

Copper Vanadium Oxide Yolk-Shell Microspheres with Excellent Capacitance and Cycling Performance for Electrochromic Supercapacitor

Vanadium pentoxide (V2O5) is considered a promising material for electrochromic supercapacitors due to its rich color transitions and excellent electrochemical capacity. However, V2O5 exhibits low electrical conductivity, and its volume changes dramatically during charge-discharge cycles, leading to structural collapse and poor long-term cyclability. These issues have hindered the development and application of V2O5. In this study, copper vanadium oxide yolk-shell microspheres (CVO) were synthesized through a one-step solvent heat treatment with an annealing process. With the doping of copper element, the capacitance, conductivity, and cyclic stability of CVO microspheres were significantly enhanced. Subsequently, the sphere-wire network structure was formed by blending Na2V6O16·3H2O nanowires (NVO), resulting in the formation of CVO/NVO composites. The three-dimensional sphere-wire network efficiently facilitates the acquisition of additional redox sites and strengthens the material-to-substrate bonding. Under the combined influence of these favorable factors, CVO/NVO achieved a high specific capacitance of 39.2 mF cm-2, with a capacitance retention of 84% after 7500 cycles at a current density of 0.7 mA cm-2. The fully inorganic solid-state electrochromic supercapacitor (ECSC), assembled on the basis of CVO/NVO, demonstrates a vivid and clearly distinguishable color change (ΔE* = 37). Even more impressive is the energy storage capacity (18.4 mF·cm-2) and the cycling stability (up to 89% retention after 10,000 cycles) exhibited by the devices. These key performances are superior to those of most of the previously reported V2O5-based ECSCs, opening a promising avenue for the development of V2O5-based electrochromic energy storage devices.

Homogeneous and Nanogranular Prussian Blue to Enable Long-Term-Stable Electrochromic Devices

The increasing demand for the state-of-the-art electrochromic devices has received great interest in synthesizing Prussian blue (PB) nanoparticles with a uniform diameter that exhibit excellent electrochromism, electrochemistry, and cyclability. Herein, we report the controllable synthesis of sub-100 nm PB nanoparticles via the coprecipitation method. The diameter of PB nanoparticles can be modulated by adjusting the reactant concentration, the selection of a chelator, and their purification. The self-assembled nanogranular thin films, homogeneously fabricated by using optimized PB nanoparticles with an average diameter of 50 nm as building blocks via the blade coating technique enable excellent performance with a large optical modulation of 80% and a high coloration efficiency of 417.79 cm2 C-1. It is also demonstrated by in situ and ex situ observations that the nanogranular PB thin films possess outstanding structural and electrochemical reversibility. Furthermore, such nanogranular PB thin films can enjoy the enhanced long-term cycling stability of the PB-WO3 complementary electrochromic devices having a 91.4% optical contrast retention after 16,000 consecutive cycles. This work provides a newly and industrially compatible approach to producing a complementary electrochromic device with extraordinary durability for various practical applications.

Transparent metal oxide interlayer enabling durable and fast-switching zinc anode-based electrochromic devices

Growing energy and environmental challenges have imposed higher requirements for the development of novel multifunctional energy storage and energy-saving devices. Electrochromic devices having similar configurations and working mechanisms with secondary batteries exhibit promising applications in dual-functional electrochromic-energy storage (ECES) devices. Electrochromic Prussian blue (PB) as typical battery cathodes are of great interest for ECES devices although they suffer from poor stability and limited capacity. In this study, a transparent metal oxide (NiO nanosheets) interlayer was incorporated to enhance the structural stability and capacity of PB while offering enlarged optical modulation (ΔT) and accelerated switching kinetics in the NiO/PB film. Impressively, the NiO/PB nanocomposite film exhibited a high areal capacity of 50 mA h m-2 and excellent electrochemical stability, simultaneously manifesting a large ΔT (73.2% at 632.8 nm), fast switching time (tc = 1.4 s, tb = 2.6 s) and higher coloration efficiency (CE = 54.9 cm2 C-1), surpassing those of the bare PB film (ΔT = 69.1% at 632.8 nm, tc = 1.6 s, tb = 4.1 s, CE = 50.9 cm2 C-1). Finally, a prototype zinc anode-based electrochromic device assembled with NiO/PB nanocomposite film exhibited a self-bleaching function and ΔT retention of up to 92% after 1000 cycles, and a 100 cm2 large area device was also demonstrated for high performance. Such a transparent metal oxide interlayer has enabled the construction of durable and fast-switching dual-functional zinc anode-based electrochromic devices and will inspire more efforts in designing novel multifunctional ECES devices.

High-Performance and Stability Electrochromic Devices with a Water Isotopologue

In this report, an ammonium metatungstate (AMT) and ferrous chloride [Fe(II)Cl2] electrochromic liquid (ECL) was synthesized using a hydrothermal method, with D2O used as the solvent instead of H2O. The results show that the use of D2O can improve the stability and performance of ECLs. The hydrogen evolution process in electrochromic devices (ECDs) filled with ECL becomes more difficult, while the material exchange process becomes easier. The ECD exhibits a color modulation amplitude of 58%@680 nm at 2 V. After 500 cycles, the device's performance remains above 95% at a current density of 1.5 mA/cm2. Hydrogen bonds in D2O solutions are expected to exhibit stronger forces compared to those in regular H2O solutions. Therefore, we hypothesize that enhancing the strength of hydrogen bonds in H2O solutions is an effective approach for improving the performance and stability of electrochromic solutions.

High-Voltage Pulse-Assisted Operation of Single-Layer Electrochromic Systems for High Performance and Reliability

A single-layer electrochromic device (SL-ECD) based on ionic conductors containing EC chromophores provides a very simple platform that can be readily fabricated by sandwiching the EC layer between two electrodes. The operation of SL-ECDs is governed by the diffusion of redox species due to their SL structure, which causes a relatively slow dynamic response. In this study, we propose an effective high-voltage pulse injection strategy to improve the performance of SL-ECDs. Applying a programmed voltage wave composed of DC and high-voltage pulses promotes coloration/bleaching switching without degrading device stability, which is more advantageous than applying high DC voltages. We modified the input voltage profile by considering fundamental parameters, such as the amplitude and duty ratio of additional voltage pulses. The coloration and bleaching dynamic responses with the optimized voltage wave are ∼62 and ∼20% faster, respectively, compared with those with the simple DC input. Furthermore, the additionally injected pulse aids in increasing the coloration efficiency from ∼95.3 to ∼168.6 cm2 C-1. Another notable feature of this system is that the device operates stably when a programmed voltage wave is used. These results indicate that the concept of high-voltage pulse-assisted operation of SL-ECDs is a straightforward but effective method for improving device performance without changing the EC chromophore or device structure.

Decoupling Electrochromism and Energy Storage for Flexible Quasi-Solid-State Aqueous Electrochromic Batteries with High Energy Density

Currently, reported aqueous electrochromic batteries (ECBs) show only limited capacity with insufficient energy density and power density. Such a limitation is naturally imposed by the rationale that the cathode of ECBs stores charge by an ion intercalation/deintercalation mechanism, where the inherent inhibition of ion diffusion and structural collapse of cathode materials through repetitive charge/discharge cycles lead to low areal capacity and unsatisfactory electrochemical performance with short lifetime. Herein, we decouple the dual functions of electrochromism and energy storage in conventional cathodes of ECBs by introducing a polyaniline/triiodide composite cathode that is in situ formed by direct electrolysis of an iodide-based quasi-solid-state aqueous electrolyte during charging. When paired with a zinc metal anode, the composite cathode can synergistically utilize the electrochromic property of polyaniline, the high-efficiency energy storage of the Zn-I2 system, as well as the effective anchorage of polyiodide by polyaniline to suppress the shuttle effect of triiodide. By selecting 1-butyl-3-methylimidazolium ion (BMI+) as the cation, a liquid-solid cathode/quasi-solid-state electrolyte interface can be achieved to facilitate the interfacial charge transfer, rendering quasi-solid-state aqueous electrochromic batteries with a high areal capacity of 1363 μAh cm-2, energy density of 1650 μWh cm-2, and power density of 5186 μW cm-2.

An Ultrafast, Energy-Efficient Electrochromic and Thermochromic Device for Smart Windows

Smart windows nowadays undertake the esteemed obligation of reducing energy consumption as well as upgrading living experience. This project aims to devise a smart window that responds to both electricity and heat, with the intention of achieving energy efficiency, privacy preservation, and enhanced decorative attributes. Through the implementation of a novel electrochromic material design, coupled with the optimization of electrochromic devices (ECDs), a high-performance ECD is obtained, demonstrating coloring/bleaching time of 0.53/0.16 s, a transmittance modulation of 78% (from 99% to 21%), and superior performance in six dimensions. Furthermore, temperature-responsive units and an ionic liquid are incorporated into the electrolyte system to create a novel thermochromic gel electrolyte with transmittance modulation from 80% to 0%, and excellent thermal insulation (6.4 °C reduction). Ultimately, an electro- and thermochromic device is developed, featuring an ultrafast color-switching speed of 0.82/0.60 s and multiple working modes. Overall, this work showcases a prospective design pathway for the development of next-generation ultrafast-switching, and energy-efficient intelligent windows.

Fabrication of Large-Area, Affordable Dual-Function Electrochromic Smart Windows by Using a Hybrid Electrode Coated with an Oxygen-Deficient Tungsten Oxide Ultrathin Porous Film

Electrochromic (EC) devices are not commercialized extensively owing to their high cost. The best large-area devices in the market suffer from not reaching a distinct dark-colored state. These devices appear more like a blue tinted glass. While a better performance demands the use of appropriate components, the cost-effectiveness of such components is crucial for commercialization. Specifically, the utilization of cost-effective electrodes, thin WO₃ coatings, and inexpensive electrolytes are essential for reducing the cost of EC devices. Here, we report a high-performing porous WO₃ thin film (∼130 nm) achieved by optimizing the DC sputtering process parameters. This way, an affordable dual-function EC energy-storage device was fabricated, showing 84% transmittance modulation and a high power density of 3036 mW/m², thus functioning simultaneously as a transparency switching energy-storage device. With a large-area (900 cm²) device, we have demonstrated that the need for expensive ITO electrodes and Li⁺ ion-based electrolytes can be eliminated by using a hybrid electrode (ITO/Al-mesh) and multivalent Al³⁺ ion-based electrolytes while not compromising the device performance. The findings of this study may revolutionize the EC device industry and their commercialization owing to inexpensive ingredients and scalable processing.

Flexible Inorganic All-Solid-State Electrochromic Devices toward Visual Energy Storage and Two-Dimensional Color Tunability

Multicolor display has gradually become a sought-after trend for electrochromic devices due to its broadened application scope. Meanwhile, the advantages of inorganic electrochromic devices such as stable electrochemical performance and good energy storage ability also have great attraction in practical production applications. However, there are still huge challenges for inorganic electrochromic materials to achieve multicolor transformation due to their single-color hue change. Herein, we design an inorganic and multicolor electrochromic energy storage device (MEESD) exhibiting flexibility and all-solid-state merits. Prussian blue (PB) and MnO₂, as the asymmetrical electrodes of this MEESD, show good pseudocapacitance property, matching charge capacity, and obvious color change. As a typical electrochromic device, the MEESD shows a fast response of 0.5 s and good coloration efficiency of 144.2 cm²/C. As an energy storage device, the MEESD presents excellent rate capability and volumetric energy/power density (84.2 mWh cm^-3/23.3 W cm^-3). Its energy level can be visually monitored by color contrast and optical modulation. In the charging/discharging process, its color can obviously change to various degrees of yellow, green, and blue along with 40% wide optical modulation at 710 nm. Meanwhile, the stability of the MEESD in a common and humidity environment was analyzed in detail from electrochemical, optical, and energy storage aspects. This work provides feasible thoughts to design multifunctional electrochromic devices integrated with inorganic, flexible, all-solid-state, multicolor, and energy storage properties.

Porous polyoxotungstate/MXene hybrid films allowing for visualization of the energy storage status in high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) have recently received growing attention for potential smart energy storage components in intelligent electronics. However, in the development of ECSCs, the design and assembly of high-performance electrode materials remain ongoing challenges. In this study, Ti₃C₂T_x MXene and polyoxotungstate (P₂W₁₈) were deposited on TiO₂ nanowires to construct a unique three-dimensional (3D) porous hybrid film, NW@MXene/P₂W₁₈, via a convenient layer-by-layer self-assembly approach. The 3D porous structure of the nanocomposite reduced the aggregation and stacking of Ti₃C₂T_x MXene nanosheets during self-assembly, leading to the formation of unobstructed ion diffusion channels and interfacial charge transfer between adjacent layers, resulting in a good electrochemical performance. Compared to the tightly packed structure, the porous hybrid film demonstrated an enhanced electrochromic energy storage performance with a higher areal capacitance (i.e., 19.0 mF cm^-2 at a current density of 0.6 mA cm^-2), in addition to a high cycling stability (i.e., 90.7% retention rate after 2000 cycles), and an excellent color rendering efficiency. Subsequently, an asymmetric ECSC was fabricated using an NW@MXene/P₂W₁₈ film as the cathode and a TiO₂ nanowire film as the anode. This ECSC exhibited a high areal capacitance of 4.0 mF cm^-2 at a current density of 0.1 mA cm^-2 with a wide operating window of 4.5 V, whilst also achieving high-speed color switching between olive green and dark blue during the charge/discharge processes, ultimately offering new avenues for the development of electrochromic energy storage electrode materials and the design of novel devices.

A Low Driving-Voltage Hybrid-Electrolyte Electrochromic Window with Only Ferreous Redox Couples

Even after decades of development, the widespread application of electrochromic windows (ECW) is still seriously restricted by their high price and inadequate performance associated with structural/fabrication complexity and electrochemical instability. Herein, a simple hybrid electrochromic system based on PFSA (perfluorosulfonic acid)-coated Prussian blue (PB, Fe4III [FeII(CN)6]3) film and Ferricyanide-Ferrocyanide ([Fe(CN)6]4-/[Fe(CN)6]3-)-containing hybrid electrolyte is reported. The PB film and the [Fe(CN)6]4-/[Fe(CN)6]3- couple show near redox potentials well inside the electrochemical window of water, resulting in a low driven voltage (0.4 V for coloring and -0.6 V for bleaching) and a relatively long lifespan (300 cycles with 76.9% transmittance contrast retained). The PFSA layer, as a cation-exchange structure, significantly improves the transmittance modulation amplitude (ΔT: 23.3% vs. 71.9% at a wavelength of 633 nm) and optical memory abilities (ΔT retention: 10.1% vs. 67.0% after 300 s open-circuit rest increases) of the device, by means of preventing the direct contact and charge transfer between the PB film and the [Fe(CN)6]4-/[Fe(CN)6]3- couple. This "hybrid electrolyte + electron barrier layer" design provides an effective way for the construction of simple structured electrochromic devices.

Recent progress in electrochromic energy storage materials and devices: a minireview

Integration of several functionalities into one isolated electrochemical body is necessary to realize compact and tiny smart electronics. Recently, two different technologies, electrochromic (EC) materials and energy storage, were combined to create a single system that supports and drives both functions simultaneously. In EC energy storage devices, the characteristic feature of EC materials, their optical modulation depending on the applied voltage, is used to visually identify the stored energy level in real time. Moreover, combining energy-harvesting and EC storage systems by sharing one electrode facilitates the realization of further compact multifunction systems. In this minireview, we highlight recent groundbreaking achievements in EC multifunction systems where the stored energy levels can be visualized using the color of the device.

Novel Prussian White@MnO2-Based Inorganic Electrochromic Energy Storage Devices with Integrated Flexibility, Multicolor, and Long Life

Flexible electrochromic devices have attracted considerable attention in recent years due to their great potential in smart multifunction electrochromic energy storage devices and wearable intelligent electronics. Herein, we present an inorganic flexible Li-based electrochromic energy storage device (EESD) by combining a Prussian white@MnO₂-composited electrode (PWM) and sputtering-made WO₃ electrode. The synergistic effect of Prussian white and MnO₂ plays a positive role both in energy storage and electrochromic property of the EESD. Its energy level can be quantified by the transmittance spectrum and chrominance difference, and its charging-discharging process can be monitored in real time by optical modulation at special wavelength. Specifically, the EESD can endure a 10,000 times cyclic voltammetry cycle without obvious degradation at wide voltage windows (-2 to 2.5 V) and realize a high coloration efficiency (77.6 cm²/C) with 35% optical modulation at 510 nm. In terms of energy storage performance, the EESD demonstrates excellent volumetric energy/power density (1.25 W cm^-3/13.2 mWh cm^-3) and remarkable stability with close to 98.3% capacitance retention and 99.4% coulombic efficiency after more than 4000 cycles. Its charging and discharging degree can be visualized in different spectral regions. There are 40% transmittance change for charging in the blue light region (450-480 nm) and 45% transmittance change for discharging in the red light region (620-750 nm). Based on its multicolor property, a quantitative indicator of charge state is achieved by the linear dependence of real-time chrominance change as stored or released charge. The ∼11 mC/cm² stored charge capacity can cause an ∼11 increase in chrominance difference ΔE value, while ∼7 mC/cm² discharge capacity can cause a ΔE value increase of ∼4. This work provides an efficient strategy to develop portable multicolor-integrated EESDs toward high performance and long stability.

Microwave Synthesized 2D WO3 Nanosheets for VOCs Gas Sensors

As an n-type semiconductor material, tungsten oxide (WO₃) has good application prospects in the field of gas sensing. Herein, using oxalic acid (OA), citric acid (CA) and tartaric acid (TA) as auxiliary agents, three homogeneous tungsten oxide nanosheets were prepared by the rapid microwave-assisted hydrothermal method. The potential exhaled gases of various diseases were screened for the gas sensitivity test. Compared with WO₃-OA and WO₃-TA, WO₃-CA exhibits significant sensitivity to formaldehyde, acetone and various alkanes. Photoluminescence (PL) chromatography and photoelectric properties show that its excellent gas sensitivity is due to its abundant oxygen vacancies and high surface charge migration rate, which can provide more preferential reaction sites with gas molecules. The experiment is of great significance for the sensor selection of the large disease exhaled gas sensor array.

Extraordinarily Stable Aqueous Electrochromic Battery Based on Li4Ti5O12 and Hybrid Al3+/Zn2+ Electrolyte

Aqueous electrochromic battery (ECB) is a multifunctional technology that shows great potential in various applications including energy-saving buildings and wearable batteries with visible energy levels. However, owing to the mismatch between traditional electrochromic materials and the electrolyte, aqueous ECBs generally exhibit poor cycling stability which bottlenecks their practical commercialization. Herein, we present an ultrastable electrochromic system composed of lithium titanate (Li₄Ti₅O₁₂, LTO) electrode and Al³⁺/Zn²⁺ hybrid electrolyte. The fully compatible system exhibits excellent redox reaction reversibility, thus leading to extremely high cycling stabilities in optical contrast (12 500 cycles with unnoticeable degradation) and energy storage (4000 cycles with 82.6% retention of capacity), superior electrochromic performances including high optical contrast (∼74.73%) and fast responses (4.35 s/7.65 s for bleaching/coloring), as well as excellent discharge areal capacity of 151.94 mAh m^-2. The extraordinary cycling stability can be attributed to the robust [TiO₆] octahedral frameworks which remain chemically active even upon the gradual substitution of Li⁺ with Al³⁺ in LTO over multiple operation cycles. The high-performance electrochromic system demonstrated here not only makes the commercialization of low-cost, high-safety aqueous-based electrochromic devices possible but also provides potential design guidance for LTO-related materials used in aqueous-based energy storage devices.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Application of Tungsten-Oxide-Based Electrochromic Devices for Supercapacitors

For making full use of the discoloration function of electrochromic (EC) devices and better show the charge and discharge states of supercapacitors (SCs), electrochromic supercapacitors (ECSCs) have attracted much attention and expectations in recent years. The research progress of tungsten-oxide-based electrochromic supercapacitors (ECSCs) in recent years is reviewed in this paper. Nanostructured tungsten oxide is widely used to facilitate ion implantation/extraction and increase the porosity of the electrode. The low-dimensional nanostructured tungsten oxide was compared in four respects: material scale, electrode life, coloring efficiency, and specific capacitance. Due to the mechanics and ductility of nano-tungsten oxide electrodes, they are very suitable for the preparation of flexible ECSCs. With the application of an organic protective layer and metal nanowire conductive electrode, the device has higher coloring efficiency and a lower activation voltage. Finally, this paper indicates that in the future, WO3-based ECSCs will develop in the direction of self-supporting power supply to meet the needs of use.

Improvement of the Interface between the Lithium Anode and a Garnet-Type Solid Electrolyte of Lithium Batteries Using an Aluminum-Nitride Layer

The next generation of all-solid-state batteries can feature battery safety that is unparalleled among conventional liquid batteries. The garnet-type solid-state electrolyte Li₇La₃Zr₂O₁₂ (LLZO), in particular, is widely studied because of its high Li-ion conductivity and stability in air. However, the poor interface-contact between Li and the electrolyte (garnet) severely limits the development of solid electrolytes. In this study, we synthesize cubic phase Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) using a secondary sintering method. In addition, a thin aluminum nitride (AlN) layer is introduced between the metal (Li) and the solid electrolyte. Theoretical calculations show that AlN has a high affinity for Li. Furthermore, it is shown that the AlN coating can effectively reduce the interface impedance between Li and the solid electrolyte and improve the lithium-ion transport. The assembled symmetric Li cells can operate stably for more than 3600 h, unlike the symmetric cells without AlN coating, which short-circuited after only a few cycles. The hybrid solid-state battery with a modified layer, which is assembled using LiFePO₄ (LFP), still has a capacity of 120 mAh g⁻¹ after 200 cycles, with a capacity retention rate of 98%. This shows that the introduction of an AlN interlayer is very helpful to obtain a stable Li/solid-electrolyte interface, which improves the cycling stability of the battery.

Improved diffusion and storage of lithium ions via recrystallization induced conducting pathways in a Li:Ta2O5-based electrolyte for all-solid-state electrochromic devices with enhanced performance

In this study, we have investigated the improvements in the performance of an all-solid-state complementary electrochromic device (ECD) by using the proposed high pressure treatment (HPT). The Li:Ta₂O₅electrolyte layer was recrystallized by the HPT utilizing pressurized CO₂gas (∼200 atm) and at low temperature (<60 °C), which enhanced the coloration performance of the WO₃/Li:Ta₂O₅/NiO complementary ECD by ∼20%. The reliability and durability of the ECD were confirmed by long term transmittance retention measurements, which indicated an improvement in the coloration performance by ∼14% upon the release of the bias voltages. The ability of the devices that were fabricated with and without the HPT process to withstand high temperature environments was also verified. In addition, photoluminescence (PL) and transmittance measurements were carried out to examine the effects of the bonding between WO₃and NiO. To determine the differences in lithium-ion (Li⁺) injection, electrical measurements were performed by utilizing varying pulse rising speeds to confirm device characteristics. The materials were characterized in terms of their composition and structure using high-resolution transmission electron microscopy along with energy-dispersive x-ray spectroscopy. Finally, a mechanistic model has been proposed to explain the improved EC characteristics based on the amorphous to crystalline transition accompanying the HPT process.

Recent Advances in Inorganic Electrochromic Materials from Synthesis to Applications: Critical Review on Functional Chemistry and Structure Engineering

For the assembly of electrochromic devices (ECDs) generally with multilayer structures, supportive components usually are needed to be incorporated with EC materials. The reasonable project and development of ECDs will achieve broad expected applications. In this study, we reviewed several impressive methods to design and fabricate ECDs with high-performance and versatility based on recent frontier research. The first part of the review is centered on the desirability and strengthening mechanism of nanostructured inorganic EC materials. The second part illustrates the recent advances in transparent conductors. We then summarize the demands and means to modify the formation of electrolytes for practicable ECDs. Moreover, efforts to increase the compatibility with the EC layer and ion capacity are delineated. In the end, the application prospects of inorganic ECDs are further explored, which offers a guideline for the industrialization process of ECDs.

Novel High-Performance and Low-Cost Electrochromic Prussian White Film

Prussian white (PW), due to its low cost, easy synthesis, open structure, and fast ion extraction/interaction, is introduced to the electrochromic field. The PW films were successfully grown on indium tin oxide (ITO) glass by a facial hydrothermal method. Impressively, the PW film exhibits excellent electrochemical cycling stability without obvious decay over 10 000 cycles and a high coloration efficiency of 149.3 cm² C^-1. The film also provides the large optical transmittance contrast (over 70%) in a wide wavelength range of 650-800 nm. Furthermore, the PW film shows the rapid coloration and bleaching response. These results suggest that PW is a promising practical candidate of high-performance electrochromic material.

Controllable Electrodeposition Adjusts the Electrochromic Properties of Co and Mo Co-Modified WO3 Films

Metal ion modification is considered to be an effective way to construct metal oxides with specific physical and chemical properties. In this paper, we prepare a tungsten oxide (WO3) film co-modified by Co-ion and Mo-ion to serve as the electrochromic material through a one-step electrodeposition method. The effect of electrodeposition time on film thickness, surface morphology and electrochromic properties is systematically studied as well. The results show that, compared with pure WO3 film, the surface morphology of the tungsten oxide film modified by Co-ion and Mo-ion (WO3: Co, Mo) is significantly different. The Co and Mo co-modified film possesses a higher transmission modulation (58.5% at 600 nm) and rapid switching speed (coloring and bleaching time are 2.7 s and 5.6 s, respectively), low impedance value and excellent cycle stability. The performance enhancement is mainly attributed to the coral-like structure of the membrane, which provides a larger specific surface area, more ion adsorption sites and faster ion diffusion. Therefore, this work provides a fast and low-cost method to prepare tungsten oxide electrochromic films co-modified with cobalt and molybdenum ions. At the same time, it also provides an idea to obtain films with different electrochromic properties by adjusting the film thickness.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

Fabrication of three-dimensional WO3 nanotube bundles on carbon cloth as a binder-free electrode for high-performance supercapacitors

The WO3 nanotube bundles are fabricated on carbon cloth, exhibiting high specific capacitance, low charge transfer resistance, and excellent stability.

Optimal Rule-of-Thumb Design of Nickel-Vanadium Oxides as an Electrochromic Electrode with Ultrahigh Capacity and Ultrafast Color Tunability

The use of electrodes capable of functioning as both electrochromic windows and energy storage devices has been extended from green building development to various electronics and displays to promote more efficient energy consumption. Herein, we report the electrochromic energy storage of bimetallic NiV oxide (NiVO) thin films fabricated using chemical bath deposition. The best optimized NiVO electrode with a Ni/V ratio of 3 exhibits superior electronic conductivity and a large electrochemical surface area, which are beneficial for enhancing electrochemical performance. The color switches between semitransparent (a discharged state) and dark brown (a charged state) with excellent reproducibility because of the intercalation and deintercalation of OH^- ions in an alkaline KOH electrolyte. A specific capacity of 2403 F g^-1, a coloration efficiency of 63.18 cm² C^-1, and an outstanding optical modulation of 68% are achieved. The NiVO electrode also demonstrates ultrafast coloration and bleaching behavior (1.52 and 4.79 s, respectively), which are considerably faster than those demonstrated by the NiO electrode (9.03 and 38.87 s). It retains 91.95% capacity after 2000 charge-discharge cycles, much higher than that of the NiO electrode (83.47%), indicating that it has significant potential for use in smart energy storage applications. The superior electrochemical performance of the best NiVO compound electrode with an optimum Ni/V compositional ratio is due to the synergetic effect between the high electrochemically active surface area induced by V-doping-improved redox kinetics (low charge-transfer resistance) and fast ion diffusion, which provides a facile charge transport pathway at the electrolyte/electrode interface.

Polymeric Ion Conductors Based on Sono-Polymerized Zwitterionic Polymers for Electrochromic Supercapacitors with Improved Shelf-Life Stability

Monolithic electrochromic supercapacitors (ECSs) have attracted increasing interest in recent electrochemical electronics due to their simplicity and unique ability to visually indicate stored energy levels. One crucial challenge for practical use is the improvement of shelf-life. Herein, zwitterionic (ZI) ionogels are proposed as effective electrolytes to reduce the self-discharging decay of ECSs. All-in-one ZI electrochromic (EC) gels are produced by one-pot sono-polymerization. The presence of ZI moieties in the gel does not affect the EC characteristics of chromophores. In addition, excellent capacitive properties in areal capacitance and coulombic efficiency are presented owing to the alignment of ZI units under an electric field and the formation of ion migration channels where rapid ion transport is allowed. Furthermore, the shelf-life of the ZI gel-based ECS is significantly improved by adjusting the interaction between polymeric gelators and ion species. The ZI gel-based ECS is expected to be a key platform for future smart energy storage devices.

Advances in Electrochemical Energy Devices Constructed with Tungsten Oxide-Based Nanomaterials

Tungsten oxide-based materials have drawn huge attention for their versatile uses to construct various energy storage devices. Particularly, their electrochromic devices and optically-changing devices are intensively studied in terms of energy-saving. Furthermore, based on close connections in the forms of device structure and working mechanisms between these two main applications, bifunctional devices of tungsten oxide-based materials with energy storage and optical change came into our view, and when solar cells are integrated, multifunctional devices are accessible. In this article, we have reviewed the latest developments of tungsten oxide-based nanostructured materials in various kinds of applications, and our focus falls on their energy-related uses, especially supercapacitors, lithium ion batteries, electrochromic devices, and their bifunctional and multifunctional devices. Additionally, other applications such as photochromic devices, sensors, and photocatalysts of tungsten oxide-based materials have also been mentioned. We hope this article can shed light on the related applications of tungsten oxide-based materials and inspire new possibilities for further uses.

Heteroatom-Doped Nickel Oxide Hybrids Derived from Metal-Organic Frameworks Based on Novel Schiff Base Ligands toward High-Performance Electrochromism

The tenets of coordination chemistry enable researchers to design and develop nanostructured materials based on metal-organic frameworks (MOFs). Herein, for the first time, we applied the Schiff base system to MOF derivatives as a strategy for the heteroatom introduction into carbon-based metal oxides toward electrochromic applications. The presented Ni-MOF thin films based on Schiff base ligands were prepared by a facile and economical reductive electrosynthesis approach, facilitating the scalable fabrication of large-size electrochromic films derived from MOFs. After the pyrolysis, the desired N-doped NiO@C (N-C@NiO) films can achieve a high cycling stability (500 cycles with 7% contrast attenuation) and coloration efficiency (80.18 cm²/C) via different pyrolysis procedures. In addition, the one-step fabricated N-C@NiO shows an excellent ability of contrast modulation (68%@580 nm) with merely 3.6% transmittance at the colored state. These improvements in electrochromic properties are attributed to hierarchical porous heterostructures and influenced by the N/C ratio and C-N bonding configuration, indicating that N-C@NiO systems derived from Schiff base MOFs are promising for low-transmittance displays.

Lithium-Ion-Assisted Ultrafast Charging Double-Electrode Smart Windows with Energy Storage and Display Applications

Lithium-ion-assisted ultrafast charging double-electrode smart windows with energy storage and a fluorescence display device (FTO/PB/Ru@SiO2||Ru@SiO2/WO/FTO) based on double electrochromic electrodes (cathode and anode) (FSDECEs) have been designed and fabricated. Here, Prussian blue (PB) and WOred are selected as the electrochromic cathode and anode, respectively. There is a synergistic effect and a large potential difference between the two electrodes. They could be simultaneously and rapidly bleached after being connected with each other. Also, the fluorescence intensity of Ru@SiO2 nanoparticles (NPs) could be regulated by the fluorescence resonance energy transfer effect (FRET). After discharging, the two electrochromic electrodes in the bleached state can be recharged by a Mg-O2 battery with a FeN5 single atomic catalyst to quickly recover the colored state. The double electrochromic electrodes can reversibly alter between coloring and bleaching states only by connecting and disconnecting the electrodes. The fluorescence intensity of FSDECEs can switch between quenching and emission, thus endowing the "on" and "off" functions. The system is concise, environmentally friendly, and easy to operate. The proposed FSDECEs demonstrate high fluorescence contrast, a fast response time, and long-term stability. Such an ingenious design of fluorescence switching based on the double electrochromic electrode in a single cell sheds light on next-generation transparent, portable, and self-powered electrochromic devices and electronic equipment.

An improvement in the coloration properties of Ag deposition-based plasmonic EC devices by precise control of shape and density of deposited Ag nanoparticles

Ag nanoparticles exhibit various colors depending on their localized surface plasmon resonance (LSPR). Based on this phenomenon, Ag deposition-based electrochromic devices can represent various optical states in a single device such as the three primary colors (cyan, magenta, and yellow), silver mirror, black and transparent. A control of the morphology of Ag nanoparticles can lead to dramatic changes in color, as their size and shape influence the LSPR band. In this research, we focused on the diffusion rate of Ag⁺ ions when Ag nanoparticles are electrochemically deposited. Consequently, well-isolated Ag nanoparticles were obtained due to the slow growth rate by using an electrolyte with a low concentration of Ag⁺ ions, resulting in an improvement in the color quality of cyan and magenta. Additionally, spherical Ag nanoparticles were deposited in the same device by optimizing their voltage application conditions, which represented yellow and green colors. In particular, green coloration is a unique phenomenon because it can appear by the combination of two absorption peaks of LSPR. As a result of investigating the finite-difference time-domain method, it was observed that the LSPR band in the long wavelength region was originated from the effects of the connection between Ag particles.

An Efficient Electrochromic Supercapacitor Based on Solution-Processable Nanoporous Poly{tris[4-(3,4-ethylenedioxythiophene)phenyl]amine}

A new green synthetic route to tris[4-(3,4-ethylenedioxythiophene)phenyl]amine (TEPA) monomer has been developed and the molecular structure of TEPA has been determined by using single-crystal XRD. Solution-processable nanoporous poly{tris[4-(3,4-ethylenedioxythiophene)phenyl]amine} (PTEPA) is prepared by a chemical oxidative polymerization in a microemulsion. Based on the distorted structure of TEPA in the solid state, it is proposed that dendritic PTEPA has a distorted 3 D conformation with multiple twisted channels and pores that are narrowed and blocked by bifurcation and distortion of PTEPA, which is consistent with the observed hierarchical pore structure. As a cathode material, PTEPA exhibits a discharge capacity of 89.5 mAh g-1 in the initial cycle with a highly sloping two-stage discharge curve and relatively stable cycling performance. Beyond its excellent energy storage properties, PTEPA also shows relatively good electrochromic performance. Furthermore, an efficient all-solid-state electrochromic supercapacitor (ECSC) with good electrochromic performance and high energy storage capacity (13.3 mF cm-2 ) is assembled from PTEPA and nanoporous graphene films. During charge-discharge processes, the color of the ECSC changes between yellow-green and steel blue. Thus, the energy storage level of the ECSC can be monitored by the corresponding color changes. The fabricated ECSC may have practical applications, for example, in self-powered electrochromic smart windows.

An electrochromic supercapacitor based on an MOF derived hierarchical-porous NiO film

Nickel oxide (NiO) is a promising candidate for future electrochromic supercapacitors due to its pronounced electrical properties and low cost. Unfortunately, the weak interaction between NiO films and conductive substrates results in poor cycling stability. In addition, the long color-switching time and low capacitance by the small lattice spacing in dense NiO impede its practical applications seriously. Herein, a hierarchical porous NiO film/ITO glass bifunctional electrode has been prepared via the solvothermal and subsequent calcination process of growing MOF-74 in situ on ITO, which shows outstanding cycle reversibility, excellent capacitance, high coloration efficiency and short color-switching time. Because of the strong binding force between the NiO film and substrate, and large surface areas with a hierarchical porous structure which are beneficial to the ion transport, the NiO film demonstrates perfect capacitive and electrochromic properties. As a bifunctional electrode, the NiO film shows a specific capacitance of 2.08 F cm-2 at 1 mA cm-2, large optical modulation of 41.08% and about 86% of optical modulation retention after 10 000 cycles. Furthermore, we assembled a bifunctional device whose energy condition can be roughly estimated according to the color state of the device. This finding can provide us with a new application of MOFs in the dual device of electrochromic supercapacitors.

Tracking Dynamic Doping in a Solid-State Electrochromic Device: Raman Microscopy Validates the Switching Mechanism

Solid-state electrochromic devices often need appropriate characterization to establish the real working mechanism for optimization and diagnosis. Raman mapping has been used here to track "dynamic doping", an important concept in organic electronics and in polythiophene-based solid-state electrochromic devices to understand and validate the mechanism of bias-induced redox-driven color switching. The proposed method demonstrates the live formation and movement of polarons which is best suited for in situ solid-state Raman spectroelectrochemistry. A 2-fold approach has been adopted here for this (1) by fabricating a working device in cross bar geometry followed by in situ spectroscopy to demonstrate the device functioning and (2) by carrying out Raman mapping from a device in custom-designed thin-film-transistor-like geometry to track and actually "see" the mechanism spectroscopically.

Green fabrication of a complementary electrochromic device using water-based ink containing nanoparticles of WO3 and Prussian blue

We fabricated a complementary electrochromic device (ECD) by using water-dispersible nanoparticles (NP) of Prussian blue (PB) and WO₃ by using a wet process, which involved just coating. Although the ECD had a thick WO₃ film, it showed much higher contrast compared to other techniques. In addition, the ECD also showed fast optical switching speed and high durability over 100 cycles because of wettability control of NP inks.

Recent development of biofuel cell based self-powered biosensors

BFC-based SPBs have been used as power sources for other devices and as sensors for detecting toxicity and BOM.

Bifunctional electrochromic-energy storage materials with enhanced performance obtained by hybridizing TiO2 nanowires with POMs

A bifunctional electrochromic-energy storage film with enhanced performance is designed and fabricated by hybridizing TiO₂ nanowires with POMs.

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates were developed.

Novel tunneled phosphorus-doped WO3 films achieved using ignited red phosphorus for stable and fast switching electrochromic performances

Simultaneous improvement of both the performance and stability of electrochromic devices (ECDs) to encourage their practical use in various applications, such as commercialized smart windows, electronic displays, and adjustable mirrors, by tuning the film structure and the electronic structure of transition metal oxides remains a challenging issue. In the present study, we developed novel tunneled phosphorus (P)-doped WO3 films via the ignition reaction of red P. The ignited red P, which can generate exothermic energy, was used as an attractive factor to create a tunneled structure and P-doping on the WO3 films. Therefore, by optimizing the effect of ignited red P on the WO3 films, tunneled P-doped WO3 films fabricated by using 1 wt% red P demonstrated a striking improvement of the EC performances, including both a fast switching speed (6.1 s for the colouration speed and 2.5 s for the bleaching speed) caused by the improvement of Li ion diffusion by the tunneled structure and electrical conductivity by P-doping WO3 and a superb colouration efficiency (CE, 55.9 cm2 C-1) as a result of increased electrochemical activity by the elaborate formation of the tunneled structure. Simultaneously, this film displayed a noticeable long-cycling stability due to a higher retention (91.5%) of transmittance modulation after 1000 electrochromic (EC) cycles as compared to bare WO3 films, which can mainly be attributed to the optimizing effect of the tunneled structure to generate an efficient charge transfer and an alleviated structural variation during the insertion-extraction of Li ions. Therefore, our results suggest a valuable and well-designed strategy to manufacture stable fast-switching EC materials that are fit for various practical applications of the ECDs.

High-Performance Asymmetric Electrochromic-Supercapacitor Device Based on Poly(indole-6-carboxylicacid)/TiO2 Nanocomposites

A difunctional porous network of poly(indole-6-carboxylicacid) (PICA)/TiO₂ nanocomposites is first prepared using TiO₂ nanorod arrays as the scaffold. Because of the synergistic effect of PICA and TiO₂, the nanocomposites show good electrochemical performance, a high specific capacitance value (23.34 mF cm^-2), and excellent galvanostatic charge-discharge stability. Meanwhile, this nanocomposite can be reversibly switched (yellow, green, brown) with a high coloration efficiency (124 cm² C^-1). An asymmetric electrochromic-supercapacitor device (ESD) is also constructed using the PICA/TiO₂ nanocomposites as the anode material and poly(3,4-ethylenedioxythiophene) as the cathode material. This ESD has robust cycle stability and a high specific capacitance value (9.65 mF cm^-2), which can be switched from light green to dark blue. After charging, the device can light up a single LED for 108 s, and the energy storage level can also be monitored by the corresponding color changes. This constructed ESD will have great potential applications in intelligent energy storage and other smart electronic fields.

Dual-colored 4,4′,4′′,4′′′-(cyclobutane-1,2,3,4-tetrayl)-tetrabenzoate electrochromic materials with large optical contrast and coloration efficiency

This paper reports novel ester-containing electrochromic materials, 4,4′,4′′,4′′′-(cyclobutane-1,2,3,4-tetrayl)tetrabenzoate derivatives, with dual-colored electrochromism, high color contrast and coloration efficiency.